Test mass and method for interferometric gravity characteristic measurement

Abstract

A test mass used for light beam interferometric gravity characteristic measurement has a center of mass located equidistant and colinear with optical center points of two oppositely reflecting retroreflectors. Rotation of the test mass about its center of mass during freefall changes the path length of the oppositely reflected light beams by equal amounts, thereby achieving common mode cancellation of the effects of test mass rotation when the two reflected light beams are interferometrically combined.

What is claimed: 
     
       1. A test mass having a center of mass point and which is adapted to freefall under the influence of gravity in an interferometric light beam gravity characteristic measurement apparatus, comprising:
 a first retroreflector having an optical center point and operative to reflect a first incident light beam as a first reflected light beam; 
 a second retroreflector having an optical center point and operative to reflect a second incident light beam as a second reflected light beam; and wherein: 
 the first and second retroreflectors are retained on the test mass with their respective optical center points each separated at a finite equal distance from the center of mass point, with the optical center points and the center of mass point all co-linear with one another, with the center of mass point located between the optical center points, and with the first and second retroreflectors oriented to reflect the first and second reflected light beams parallel to one another and in opposite directions from one another. 
 
     
     
       2. A test mass as defined in  claim 1 , wherein:
 the first retroreflector reflects the first reflected light beam parallel to the first incident light beam; and 
 the second retroreflector reflects the second reflected light beam parallel to the second incident light beam. 
 
     
     
       3. A test mass as defined in  claim 1 , further comprising:
 a support structure which retains the first and second retroreflectors to the test mass in predetermined positions relative to each other; and 
 a weight moveably attached to the support structure to adjust the position of the center of mass point of the test mass with respect to the optical center points of the first and second retroreflectors. 
 
     
     
       4. A test mass as defined in  claim 1 , further comprising:
 a first housing element which retains the first retroreflector to the test mass in a predetermined position relative to the first housing element; 
 a second housing element which retains the second retroreflector to the test mass in a predetermined position relative to the second housing element; and wherein: 
 the first and second housing elements are moveable relative to each other to position the optical center points of the first and second retroreflectors relative to the center of mass point. 
 
     
     
       5. A test mass as defined in  claim 1 , further comprising:
 a first housing element which retains the first retroreflector to the test mass in a predetermined position; 
 a second housing element which retains the second retroreflector to the test mass in a predetermined position; and wherein: 
 the first and second housing elements surround the first and second retroreflectors, respectively; 
 the first housing element defines pass-through openings to allow the first incident and reflected light beams to impinge upon and reflect from the first retroreflector; and 
 the second housing element defines pass-through openings to allow the second incident and reflected light beams to impinge upon and reflect from the second retroreflector. 
 
     
     
       6. A test mass as defined in  claim 1  wherein:
 the first and second retroreflectors are open. 
 
     
     
       7. A test mass as defined in  claim 1 , wherein:
 each of the first and second retroreflectors is defined by three mutually perpendicular reflective wall surfaces which intersect at a corner; and 
 the corners of the first and second retroreflectors are each positioned coincidently with the co-linear relationship of the optical center points and the center of mass point. 
 
     
     
       8. A test mass as defined in  claim 7 , wherein the interferometric light beam gravity characteristic measurement apparatus is a gravimeter. 
     
     
       9. A test mass as defined in  claim 1 , wherein the interferometric light beam gravity characteristic measurement apparatus is a gradiometer. 
     
     
       10. A test mass as defined in  claim 9 , wherein the gradiometer comprises two of said test masses which are vertically separated by a distance and which freefall simultaneously to measure a gradient of gravity over the distance between the two test masses. 
     
     
       11. A method of using the test mass defined in  claim 1  in an interferometric light beam gravity characteristic measuring apparatus, comprising:
 freefalling the test mass; 
 rotating the test mass while freefalling; 
 directing a first light beam into a first beam arm which has a length, the first light beam in the first beam arm impinging upon and reflecting from the first retroreflector while the test mass is freefalling; 
 directing a second light beam into a second beam arm which has a length, the second light beam in the second beam arm impinging upon and reflecting from the second retroreflector while the test mass is freefalling; 
 changing the lengths of the first and second beam arms by an amount related to the freefall movement of the test mass; 
 changing the lengths of the first and second beam arms by an amount related to rotation of the test mass during the freefall movement; 
 combining the first and second reflected light beams into a combined light beam to create interference fringes in the combined light beam; 
 canceling interference fringes in the combined light beam arising from the first and second reflected light beams caused by rotation of the test mass during the freefall movement; and 
 determining the gravity characteristic from the interference fringes of the combined light beam after cancellation of the interference fringes caused by rotation of the test mass during the freefall movement. 
 
     
     
       12. A method of using the test mass defined in  claim 2  in an interferometric light beam gravity characteristic measuring apparatus, comprising:
 freefalling the test mass; 
 rotating the test mass while freefalling; 
 directing a first light beam into a first beam arm which has a length, the first light beam in the first beam arm impinging upon and reflecting from the first retroreflector while the test mass is freefalling; 
 directing a second light beam into a second beam arm which has a length, the second light beam in the second beam arm impinging upon and reflecting from the second retroreflector while the test mass is freefalling; 
 changing the lengths of the first and second beam arms by an amount related to the freefall movement of the test mass; 
 changing the lengths of the first and second beam arms by an amount related to rotation of the test mass during the freefall movement; 
 combining the first and second reflected light beams into a combined light beam to create interference fringes in the combined light beam; 
 canceling interference fringes in the combined light beam arising from the first and second reflected light beams caused by rotation of the test mass during the freefall movement; and 
 determining the gravity characteristic from the interference fringes of the combined light beam after cancellation of the interference fringes caused by rotation of the test mass during the freefall movement. 
 
     
     
       13. A method of eliminating adverse effects of rotation of a freely falling test mass which has a center of mass point when measuring a gravity characteristic by light beam interferometry, comprising:
 freefalling the test mass; 
 rotating the test mass during freefall; 
 respectively reflecting first and second light beams from first and second retroreflectors of the test mass during freefall, each first and second retroreflector having an optical center point, the respective optical center points of each first and second retroreflector are each spaced at a finite equal distance from the center of mass point, the optical center points and the center of mass point are located co-linearly, the center of mass point is located between the optical center points, and the first and second retroreflectors are oriented to reflect the first and second light beams parallel to one another and in opposite directions from one another; 
 combining the first and second reflected light beams interferometrically to create fringes which characterize the gravity characteristic to be measured; 
 eliminating aberrant fringes from the combined first and second reflected light beams resulting from the rotation of the test mass during freefall; and 
 determining the measured gravity characteristic from the fringes of the combined first and second light reflected beams after eliminating the aberrant fringes. 
 
     
     
       14. A method as defined in  claim 13 , wherein eliminating the aberrant fringes further comprises:
 impinging and reflecting the first and second light beams on the first and second retroreflectors in separate paths which each have a length; 
 reflecting the first and second light beams from the first and second retroreflectors attached to the test mass during rotation of the test mass during freefall; and 
 changing the path length of each first and second light beam equally when impinging on and reflecting from the first and second retroreflectors during rotation of the test mass during freefall. 
 
     
     
       15. A method as defined in  claim 14 , further comprising:
 rotating the test mass about the center of mass point during freefall. 
 
     
     
       16. A method as defined in  claim 15 , further comprising:
 reflecting the first light beam from the first retroreflector parallel to the impingement of the first light beam on the first retroreflector during rotation of the test mass during freefall; and 
 reflecting the second light beam from the second retroreflector parallel to the impingement of the second light beam on the second retroreflector during rotation of the test mass during freefall. 
 
     
     
       17. A method of using the two test masses in the gradiometer as defined in  claim 10 , wherein the two test masses are designated as a first test mass and a second test mass, respectively, and wherein the method comprises:
 freefalling the first and second test masses simultaneously; 
 rotating at least one of the first and second test masses while both of the test masses are freefalling simultaneously; 
 directing a first light beam into a first beam arm having a length, the first light beam in the first beam arm impinging upon and reflecting from a first retroreflector of the first test mass and a second retroreflector of the second test mass while both of the test masses are freefalling; 
 directing a second light beam into a second beam arm having a length, the second light beam in the second beam arm impinging upon and reflecting from a second retroreflector of the first test mass and a first retroreflector of the second test mass while both of the test masses are freefalling; 
 changing the lengths of the first and second beam arms by an amount related to any relative change in distance separating the first and second test masses during simultaneous freefall of both of the test masses; 
 changing the lengths of the first and second beam arms by an equal amount related to the rotation of the one test mass during simultaneous freefall; 
 combining the first and second reflected light beams into a combined light beam to create interference fringes in the combined light beam; 
 canceling interference fringes in the combined light beam arising from the first and second reflected light beams caused by rotation of the one test mass during simultaneous freefall; and 
 determining the gravity characteristic from the interference fringes of the combined light beam after cancellation of the interference fringes caused by rotation of the one test mass.